Lauridsen, Jonas Skjødt

Anvendelsen af aktive magnetlejer (AMB) er steget over de seneste årtier efterhåndensom teknologien er modnet og de elektriske komponenter er blevet billigere ogbedre. AMB’erne er velegnet til blandet andet turbo-maskiner grundet deres mange fordelesom ingen mekanisk kontakt, intet smøringsbehov, lave vedligeholdelsesomkoster,små vibrationer, høj rotationshastighed og lavt energiforbrug. Disse fordele gør AMB’ersærdeles brugbare til udfordrende applikationer som f.eks. undersøisk turbomaskinerfor olie- og gas produktion, hvor pålidelighed, lav vedligehold og høj hastighed erafgørende.Tætninger er en nøglekomponent i turbomaskiner. De forhindrer intern flow-lækagefra højtryks til lavtryksområder for at forbedre den overordnede effektivitet. I mangeapplikationer påvirker de dog også de rotor-dynamiske egenskaber markant. Af dengrund skal de inkluderes i den rotor-dynamiske stabilitetsanalyse. Tætningskræfterneer i mange tilfælde svære at modellere grundet blandt andet komplekse geometrier aftætningerne og multifase-flow. I øjeblikket findes der ingen generel accepteret metodetil at bestemme dynamiske tætningskræfter. Store usikkerheder må derfor forventes vedmodellering af dynamisk tætningskræfter og som følge deraf også i den rotor-dynamiskestabilitets-analyse.Denne afhandling har fokus på i) lukket-sløjfe identifikation af usikre AMB-parametre,ii) lukket-sløjfe identifikation af ukendte stivheds- and dæmpningskoefficienter af endynamisk tætningsmodel og iii) design af AMB-regulatorer til at håndtering af usikretætningskræfter. Regulatorer der kan garantere stabilitet og performance mod usikretætningskræfter er i hovedfokus.Hovedbidraget består i et framework til design af modelbaseret regulatorer forAMB-systemer udsat for usikre og skiftende dynamiske tætningskræfter. En identifikationsmetodeer blevet udviklet og eksperimentelt valideret til at opnå præcise modellerpå LFT (Linear Fractional Transformation) form for design af H1, and LPV (LinearParameter Varying) regulatorer. Tætningsparametrene er identificeret on-site uden brugaf specialudstyr.En perturberet model af det kombinerede AMB-rotor-tætningssystem er konstrueretved brug af Finite Element Metoden (FEM), modal-reduktion og Linear FractionalTransformation (LFT). Den beskriver den dynamiske respons ved parametriske usikkerheder/ændringer af stivheds- og dæmpningskoefficienter af tætninger og usikkerheden istivhederne af AMB’erme. Ved brug af forskellige typer excitations-signaler, dvs. steppedsinus og Pseudo Random Binary Signal (PRBS), og optimering i tidsdomæne, er deovennævnte parametre identificeret. Indsættelse af de identificerede parametre i deniv Resumé (Dansk)kendte modelstruktur resulterer i præcise modeller, som simuleret, stemmer overensmed eksperimentel data. Den perturberede model er yderligere anvendt til at beskriveusikkerheder i tætningskræfterne til robust regulatorfremstilling og for Linear ParameterVarierende (LPV) regulatorfremstilling, til at kompensere for kendte ændringer itætningsdynamikken som funktion af operationsbetingelserne.Et rotor dynamisk test-facilitet bestående af en stiv rotor, to radielle AMB’er og enturbomaskine-tætning, er anvendt til i) lukket sløjfe identifikation af parametre i AMBrotormodellen,ii) identifikation af dynamiske tætningskræfter, iii) implementering afAMB-regulatorer til kompensering for dynamiske tætningskræfter. Stabilitet og ydelseaf de designede regulatorer er evalueret og sammenlignet i forhold til en decentraliseretPID referenceregulator. Regulatorer baseret på identificerede nominelle tætningsmodelleryder god kompensation for destabiliserende tætningskræfter. Yderligere kan enbetydelig forbedring i performance ses for robuste regulatorer, der kan kompenserefor ændringer i operationstryk bedre, sammenlignet med en nominel modelbaseretregulator. Simuleringer ved brug af begge typer regulatorer matcher eksperimentielleresultater.The use of Active Magnetic Bearings (AMBs) in industrial applications has increased over recent decades as the technology has grown more mature, further aided by advancements and decreasing prices of the electronic components. AMBs are well suited to turbo-machinery applications offering several advantages over traditional types of bearings, including: no mechanical contact, no lubrication, low maintenance, low vibration level, high rotational speed and low energy consumption. These advantagesmake AMBs especially useful in challenging environments, for instance in subsea turbomachinery applications for oil and gas production, where reliability, low maintenance and high speed are of great importance. Annular seals are a key component in turbomachinery. They prevent internal flow leakage from high pressure to low pressure regions and improve the overall machine efficiency; in many applications, however, they also affect the system rotor-dynamic properties significantly. For this reason, the seal characteristics must be included in the rotor-dynamic stability analysis. Unfortunately, in many cases the seal forces are hard to model due to complex geometries of the seal and multiphase fluids. At present, there is no generally accepted method for determination of dynamic seal forces. Therefore, large uncertainties must be expected when modelling dynamic seal forces and consequently also in rotor-dynamic stability analysis.This thesis focuses on i) closed loop identification of uncertain AMB parameters, ii) closed loop identification of unknown stiffness and damping coefficients of a dynamicseal model and iii) the design of AMB controllers to handle dynamic seal forces. Controllers that can guarantee stability and performance in the presence of uncertainseal forces are of special interest. The main original contribution of the thesis is the framework for design of model based controllers for AMB systems subjected to uncertainand changing dynamic seal forces. An identification method has been developed, and experimentally validated, to obtain precise models of Linear Fractional Transformation (LFT) form for synthesising H1, μ and Linear Parameter Varying (LPV) controllers. The seal parameters and AMB dynamics are identified on-site without any need ofspecial equipment.A perturbed model of the combined AMB, rotor and seal system is constructed usingFinite Element Methods (FEM), modal reduction and LFT. It describes the dynamicbehaviour due to parametric uncertainties/changes of the damping and stiffness coefficient sof the seal and the uncertainties in the stiffness of the AMBs. Using different types of excitation signals, i.e. stepped sine, impulse and Pseudo Random Binary Sequence(PRBS), and optimisation in the time domain, the above mentioned parameters areidentified. Inserting the identified parameters in the known model structure results inaccurate models, which - when simulated - fit experimental data well. The perturbedmodel is further used for the robust controller synthesis to describe the uncertaintiesin seal forces and for LPV control synthesis, to compensate for known changes in seal forces due to changes in operating conditions. A rotor dynamic test facility with a rigid rotor, two radial AMBs and one annular test seal is used for i) closed loop identification of parameters in the AMB-rotor model, ii) identification of dynamic seal forces, iii) implementation of AMB controllers to compensate for dynamic seal forces. The stability and performance of the designed controllers are examined and compared to a reference decentralised PID controller. Controllers based on identified nominal seal models are shown to provide good compensation for the destabilising dynamic seal forces. Furthermore, significant performance improvement is shown when using a robust controller, which can handle changes in operational pressures better, in comparison to a nominal model based controller. Simulations using both type of model based controllers match experiments well

Online Research Database In Technology

Control Design of Active Magnetic Bearings for Rotors Subjected to Destabilising Seal Forces - Theory &amp; Experiment

http://orbit.dtu.dk/ws/files/139513157/PhD_Thesis_Jonas_Lauridsen.pdf

Control Design of Active Magnetic Bearings for Rotors Subjected to Destabilising Seal Forces - Theory & Experiment

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Online Research Database In Technology

Control Design of Active Magnetic Bearings for Rotors Subjected to Destabilising Seal Forces - Theory &amp; Experiment

Abstract

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Full text

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Online Research Database In Technology

Control Design of Active Magnetic Bearings for Rotors Subjected to Destabilising Seal Forces - Theory & Experiment